1. Field of the Invention
The present invention relates to a radio transmission method to be preferably applied to building up a local area network (LAN) among multiple appliances by transmitting various information by means of radio signals and a radio transmission apparatus to which the same radio transmission method is applied.
2. Description of the Related Art
Conventionally, when a local area network is built up among plural appliances like various video appliances or personal computer and their peripherals in a relatively small area such as home and office so as to transmit data handled among them, radio transmission method has been sometimes used by attaching a radio signal transmitting/reception device to respective appliances instead of connecting a signal line directly among those appliances.
By building up a local area network by radio transmission, the necessity of connecting respective appliances directly with a signal line is eliminated, thereby simplifying a system configuration.
However, if signals are transmitted from plural transmitters when a local area network is built through the plural radio transmitters, there is a possibility that a transmission error may occur. Thus, it is necessary to access-control communication among the respective transmitters in the network according to some method.
As a conventionally well known access control method in a small size radio network, there is a method of controlling communications among respective transmitters (terminal stations: nodes) in the network in a unified way with a central transmitter (central control station: root node) in a star type connection. For example, communication in the network is controlled by polling control. According to this method, the central control station in the network transmits a control signal for polling to the other nodes in the network successively so that transmission from each node is carried out in order according to the polling. By transmission processing by the polling, transmission efficiency can be improved.
In the star connection type network configuration, all communication stations in the network need to be capable of communicating with the central control station directly by radio, and therefore, the network configuration is limited. Thus, it has been proposed to make a communication station incapable of communicating with the central control station directly by radio act as a terminal station (hidden terminal station) of this network by relaying a control signal or the like sent from the central control station through a particular terminal station.
The communication station set up as the central control station needs to always transmit a control signal for polling and synchronous signal and always be actuated. Then , if the above described hidden terminal station is set up, it is necessary to always transmit a control signal for polling and synchronous signal to a communication station for relaying the control signal sent from the central control station to the hidden terminal station.
Therefore, the communication station which relays the control signal or the like needs to be always actuated. Thus, there is a problem that power consumption of a radio transmission apparatus constituting that communication station is relatively large. If the radio transmission apparatus is always powered by commercial AC power supply, there is no problem. However, if an incorporated battery is used as power supply, if the relay transmission is always executed, the duration of the battery is limited to a very short time. If the capacity of the battery is exhausted, the relay transmission of this radio transmission apparatus is disabled. Thus, there is, another problem that the hidden terminal station cannot communicate with other stations.
Further, a communication station which relays the control signal or the like from the central control station needs to be disposed at a position in which it is capable of communicating with both the central control station and the hidden terminal station directly. However, some radio transmission apparatus may be so structured to be portable and that portable radio transmission apparatus may be carried away. That is, some communication station is structured as a radio transmission apparatus in a relatively small size and powered by an incorporated secondary battery. For example, such a radio transmission station may be connected to an portable electronic apparatus and always carried from one place to another place.
As the communication station for relaying to the hidden terminal station, a particular station located at a position in which it is capable of communicating with both the central control station and the hidden terminal station directly is specified according to a topology map or the like determined by the central control station, for example. If such a portable radio transmission apparatus is specified as a relay station, there is a possibility that the relay transmission may be disabled when the portable radio transmission apparatus is carried to other place, so that communication with the hidden terminal station is also disabled.
Although a case in which the control signal from the central control station is relayed to the hidden terminal station has been stated here, the same problem occurs if a signal is relayed from the hidden terminal station to other terminal station.
Accordingly, an object of the present invention is to provide a radio transmission method which enables to choose a relay station favorably when a signal from a central control station is relayed in a radio network.
To achieve the above object, according to an aspect of the present invention, there is provided a radio transmission method for carrying out radio transmission in radio transmission network comprised of plural communication stations by control of a communication station set up as a central control station, wherein at least one of the plural communication stations determines a predetermined condition of itself, determines whether or not it is capable of relaying a signal sent from other station according to the determined condition and if it is determined that the relay transmission is possible, sets up itself in a condition capable of relay transmission.
According to this radio transmission method, only if each communication station is in a condition suitable for relay transmission, the relay transmission through that station is enabled. Therefore, if each communication station is not in the condition suitable for the relay transmission, no relay transmission is carried out in this network.
According to another aspect of the present invention, there is provided a radio transmission apparatus for carrying out radio communication with other communication station in a network, comprising: a communication processing means for transmitting or receiving a radio signal; a condition determining means for determining a predetermined condition of the apparatus; and a control means for determining whether or not relay transmission is possible according to determination of the condition determining means and carrying out the relay transmission with the communication processing means according to the determination.
According to this radio transmission apparatus, only when this radio transmission apparatus is in a condition suitable for the relay transmission, the relay transmission through this transmission apparatus is enabled. Therefore, if the transmission apparatus is not in the condition suitable for the relay transmission, this transmission apparatus is set to a condition not allowing the relay transmission.